Synapsids
Synapsids – not to be confused with , which are a subordinate group to synapsids – are a group of animals that includes s and every animal more closely related to mammals than to the other members, s and , included in the s . They are easily separated from other amniotes by having a , an opening low in the behind each eye, leaving a beneath each; this accounts for their name. Primitive synapsids are usually called or pelycosaur- synapsids. This informal term consists of all synapsids which are not s, a monophyletic more advanced mammal-like group. The non-mammalian synapsids are described as mammal-like reptiles in classical systematics; they can also be called stem mammals or proto-mammals. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes, the other being the , a group that includes modern reptiles and birds. The distinctive temporal fenestra developed in the ancestral synapsid about 312 ago, during the . Synapsids were the largest in the period, 299 to 251 million years ago, although some of the larger s at the end of the Permian could match them in size. As with other groups then extant, their numbers and variety were severely reduced by the . By the time of the extinction at the end of the Permian, all the older forms of synapsids (known as pelycosaurs) were already gone, having been replaced by the more advanced therapsids. Although the s and , the latter consisting of the ( ) and ( ia), continued into the period as the only known surviving therapsids, s became the largest and most numerous land vertebrates in the course of this period. The , a discovery first reported in 2008, was the size of an elephant. The cynodont group , which includes , were the only synapsids who outlasted the Triassic. After the , the synapsids (in the form of mammals) again became the largest land animals as well as becoming the largest marine animals. The synapsids are traditionally divided into a primitive group and an advanced group, known respectively as s and s. 'Pelycosaurs' make up the six most primitive families of synapsids. They were all rather lizard-like, with sprawling gait and possibly horny . The therapsids contain the more advanced synapsids, having a more erect pose and possibly hair, at least in some forms. In traditional taxonomy, the Synapsida encompasses two distinct successively closer to mammals: the low-slung pelycosaurs have given rise to the more erect therapsids, who in their turn have given rise to the mammals. In traditional vertebrate classification, the Pelycosauria and Therapsida were both considered of the subclass Synapsida. In , the terms are used somewhat differently, as the daughter clades are included. Most papers published during the 21st century have treated "Pelycosauria" as an informal grouping of primitive members. Therapsida has remained in use as a clade containing both the traditional therapsid families and mammals. However, in practical usage, the terms are used almost exclusively when referring to the more members that lie outside of . Phylogenetic tree |label2= |2= |2= |2= |label2= |2= |label2= |2= |label2= |2= |label2= |2= |label2= |2= |label2= |2= |label2= |2= |label2= |2= ' }} }} }} }} }} }} }} }} }} }} }} }} }} References Category:Tree of life